FIGS. 8 to 10 illustrate an example of a conventional structure of an electric power steering device disclosed in Patent Document 1. The electric power steering device is configured to transmit rotation of a steering wheel 1 to an input shaft 3 of a steering gear unit 2 and to push and pull a pair of left and right tie-rods 4, 4 in association with rotation of the input shaft 3, thereby applying a steering angle to wheels. The steering wheel 1 is supported and fixed to a rear end portion of a steering shaft 5. The steering shaft 5 is rotatably supported to a cylindrical steering column 6 supported to a vehicle body with being inserted in the steering column 6 in an axial direction. A front end portion of the steering shaft 5 is connected to a rear end portion of an intermediate shaft 8 via a universal joint 7. A front end portion of the intermediate shaft 8 is connected to the input shaft 3 via a separate universal joint 9. The illustrated example is an electric power steering device configured to reduce a force, which is necessary to operate the steering wheel 1, by using an electric motor 10 as a generation source of auxiliary power. Meanwhile, in the specification, a front-rear direction refers to a front-rear direction of a vehicle, unless particularly mentioned.
The steering column 6 is configured by combining an inner column 11 and an outer column 12 so as to be expandable and contractible. The steering column 6 is supported to the vehicle body (not shown). The steering shaft 5 is configured by combining a lower shaft 13 corresponding to an input shaft in the claims and an upper shaft 14 such that torque can be transmitted and expansion and contraction can be made. The steering shaft 5 is rotatable supported to an inner diameter side of the steering column 6. The steering wheel 1 is fixed to a part of the upper shaft 14 protruding from a rear end opening of the outer column 12. A front end portion of the inner column 11 is joined and fixed with a housing 15, and a front half part of the lower shaft 13 is inserted in the housing 15.
An output shaft 16 is rotatably supported to a front side of the lower shaft 13 in the housing 15 by a pair of ball bearings 17, 18. A part of the output shaft 16 protruding from a front end opening of the housing 15 is joined with the universal joint 7. The output shaft 16 and the lower shaft 13 are coaxially coupled to each other via a torsion bar 19.
The torsion bar 19 is formed of spring steel and has a first coupling shaft part 20 and a second coupling shaft part 21 provided at both axial end portions, a spring shaft part 22 provided at an axially intermediate part, and a guide shaft part 23 provided between the first coupling shaft part 20 and the spring shaft part 22 and having a diameter larger than the spring shaft part 22. In a state where most of the torsion bar 19 except the rear end portion is arranged at an inner diameter side of the output shaft 16, the first coupling shaft part 20 is spline-fitted into a spline hole 24 formed in a radially central portion of front end side portion of the lower shaft 13 so as not to be relatively rotatable. The second coupling shaft part 21 is loosely fitted in a fitting hole 25 formed in a front end portion of the output shaft 16 and is then joined to the front end portion of the output shaft 16 so as not to be relatively rotatable by a pin 26 engaged thereto. A bush 27 is internally fitted and fixed to a rear end portion of an inner peripheral surface of the output shaft 16, and a guide shaft part 23 is supported to an inner diameter side thereof so as to be relatively rotatable.
A front end portion of the lower shaft 13 is provided with a cylindrical part 28. An inner peripheral surface of the cylindrical part 28 is provided with a female stopper part 29 having a circumferential concave-convex shape (gear wheel shape) where a diameter of an inscribed circle thereof is larger than the spline hole 24. In the meantime, a rear end portion of an outer peripheral surface of the output shaft 16 is provided with a male stopper part 30 having a circumferential concave-convex shape and an outer diameter dimension smaller than a rear end side portion thereof. The female stopper part 29 and the male stopper part 30 are concavity/convexity engaged with each other so as to be relatively rotatable within a predetermined angle range.
A rear end side portion, which is a portion adjacent to a front side of the male stopper part 30 in the axial direction, of the outer peripheral surface of the output shaft 16 is provided with a torque detection concave-convex part 31 having a circumferential concave-convex shape where a diameter of a circumscribed circle thereof is larger than the male stopper part 30. A cylindrical torque detection sleeve 32 formed of non-magnetic metal is arranged at an outer diameter side of the torque detection concave-convex part 31. A base end portion (rear end portion) of the torque detection sleeve 32 is externally fitted and fixed to the cylindrical part 28. A part ranging from a front end portion to an intermediate portion, which is arranged at the outer diameter side of the torque detection concave-convex part 31, of the torque detection sleeve 32 is formed with a plurality of substantially rectangular window holes 33, 33 arranged axially in a double-row and equally spaced in the circumferential direction. A torque detection coil unit 34 internally fitted and fixed to the housing 15 is arranged at an outer diameter side of the torque detection concave-convex part 31 and the torque detection sleeve 32.
A worm wheel 35 is externally fitted and fixed to an axially intermediate part of the output shaft 16. A worm 36 rotatably supported in the housing 15 is meshed with the worm wheel 35. An electric motor 10 is supported and fixed to the housing 15, and a motor output shaft 37 of the electric motor 10 is joined to a base end portion of the worm 36 such that torque can be transmitted.
According to the electric power steering device configured as described above, when a driver operates the steering wheel 1 to apply torque, which is a steering force, to the steering shaft 5, the torsion bar 19 is elastically distorted in correspondence to a direction and a magnitude of the torque. Accompanied by this, a circumferentially positional relation between the torque detection concave-convex part 31 and the torque detection sleeve 32 is changed, so that an impedance change occurs in a coil of the torque detection coil unit 34. Accordingly, it is possible to detect the direction and magnitude of the torque based on the impedance change. The electric motor 10 is configured to generate auxiliary power in correspondence to a detection result of the torque. The auxiliary power is increased by a worm reducer 38 configured by the worm wheel 35 and the worm 36 meshed with each other and is then applied to the output shaft 16. As a result, a force that is necessary for the driver to operate the steering wheel 1 is reduced.
In the meantime, when the high torque (steering force) is input from the steering wheel 1 to the steering shaft 5 and thus a distortion amount of the torsion bar 19 reaches one or other upper limit of a predetermined angle range, tooth parts of the female stopper part 29 and the male stopper part 30 are meshed with each other in the circumferential direction. Based on this meshing, a part of the torque is directly transmitted from the lower shaft 13 to the output shaft 16.
In order to assemble the electric power steering device as described above, the first coupling shaft part 20 provided at the rear end portion of the torsion bar 19 is first press-fitted into the spline hole 24 formed in the front end side portion of the lower shaft 13, thereby coupling the torsion bar 19 and the lower shaft 13 each other such that the torque can be transmitted. Then, the second coupling shaft part 21 provided at the front end portion of the torsion bar 19 is inserted into an inner side of the output shaft 16 from a rear end side, and the second coupling shaft part 21 is internally fitted to the fitting hole 25 formed in the front end portion of the output shaft 16 with no interference. Then, in a state where a positional relation between the lower shaft 13 and the output shaft 16 in a rotation direction is adjusted to a center position of the predetermined angle range, a radial through-hole 39 is drilled at a position at which the front end portion of the output shaft 16 and the front end portion (the second coupling shaft part 21) of the torsion bar 19 are aligned with each other. Then, the pin 26 is press-fitted in the through-hole 39, thereby coupling the front end portion of the torsion bar 19 to the output shaft 16 such that the torque can be transmitted. Alternatively, the through-hole 39 may be formed in advance, and the pin 26 may be press-fitted in the through-hole 39.
However, when assembling the electric power steering device of the conventional structure by the assembling method as described above, following problems may be caused.
That is, as shown in FIG. 11, when inserting the second coupling shaft part 21 provided at the front end portion of the torsion bar 19 into the output shaft 16 from the rear end side and bringing the output shaft 16 and the lower shaft 13 close to each other in the axial direction, the front end portion of the torque detection sleeve 32 is advanced into the outer diameter side of the torque detection concave-convex part 31 before the guide shaft part 23 of the torsion bar 19 is advanced into an inner diameter side of the bush 27. Therefore, it is difficult to secure coaxiality of the output shaft 16 and the lower shaft 13. Also, when bringing the output shaft 16 and the lower shaft 13 close to each other in the axial direction, attention should be taken such that the torque detection sleeve 32 and the torque detection concave-convex part 31 are not to contact each other. Therefore, the assembling operability is lowered, and there is room for improvement.